When electronic image data is acquired from an electronic input scanner, it is initially in an analog form. The sensing system of the raster input scanner detects the amount of light reflected from the scanned original and for a discrete area viewed, assigns the sensed light a representative analog value ranging over a continuous scale of values. Upon acquiring this data, the analog information is transformed into a digital approximation of the value, which in many applications may be represented, for example, by an 8 bit data byte, which has a resolution 256 levels of intensity. Since the data derived is indicative of various intensity levels, it must be converted into a format suitable for printing on an output device or display on a soft display device. For example, in a binary printing device, a two level output is required, while other devices allowing printing of more than two levels will require corresponding greater numbers of output levels. Generally, the 8 bit data of varying intensity is directed through a threshold device to determine whether the data for a selected location should be assigned a spot or a blank. The data value for any given pixel location is compared to a threshold value to determine whether a spot should be printed for that location, or if the location should be left blank. In simple applications, particularly for line graphics or text, a single threshold value may be applied to all the data derived by the scanner. Tonal range, contrast and other qualities of the output image may be varied by controlling the threshold conversion applied to the image.
When reproducing continuous tone graphics, such as photographs or paintings, it is often desirable to reproduce the original image grey. To print grey, the printer is directed to modulate the number of spots or intensity of spots within an area, rather than placing all spots or all blanks in the area. The result is perceived as grey. In a binary printer the density of grey is controlled by placing fewer or greater number of spots within the same unit of area. Selection of which pixels will appear as either spots or blanks is chosen on the basis of a digital halftone screen or dither matrix.
The screen is generally comprised of a processing arrangement which, for a defined area or half tone cell, provides a number of threshold values dispersed through the plurality of pixel locations within a defined matrix, not necessarily square. Each pixel location may be assigned a different threshold value to be applied to the scanner derived data during its processing for printing. If the area is to be black, all the pixels in the area will exceed the thresholds applied for all of the pixel locations throughout the area. For greys, the data will exceed the threshold values for a varying number of the pixels within the cell. Thresholding may be provided with the same process by providing a constant threshold value throughout the screen. A screen or screening function may also be provided by a mathematical process algorithm, which assigns a threshold value to a given pixel on a statistical basis without a fixed matrix. A class of processes known as error diffusion is an example of such a processing method.
Screens are designed for various output results, and may be used to emphasize dark or light in an output image. A variety of screen types are available which all produce grey, but provide different distributions for intensity levels throughout the screen area and, within screen types, contrast or thresholds may be varied. Differing screens may be selected for a variety of reasons such as optimizing reproduction of poor originals, enhancing images to show certain features, to create artistic effects, to minimize artifacts from previously screened input data such as moire or as described before, providing accurate reproduction of graphics. Screens may also provide different screen frequencies for better detail rendition, special effects, or accurate reproduction of complex and line graphics. In particular, a screen design must be different for each different printer or set-up of a printer. Images having a first screen applied may reproduce well on a first printer, but the same image screened with a second screen may reproduce better on another printer.
Other image processing arrangements offer the opportunity to vary an image and control its ultimate reproduction. Filters may be applied to the image to remove noise or the effects of previously applied image processing steps from the image. Enhancement filters may be applied to emphasize high frequency image data, perhaps at the expense of other features of the image.
Printers, by nature, have reproduction characteristics which may vary significantly, even among printers of identical manufacture. Such variations have the effect of varying the desired output by printing darkly or lightly the output information. These differences may be exacerbated by using certain screens on the scanner. For example, a printer which is tending to print very lightly may have considerable problems printing an image with a large amount of light print areas and slight variations of contrast. These gradations simply may not be printed, causing an undesirable quality of printing. A potential remedy to the problem may be to apply a different screen which may print a more contrasted image. If moire or aliasing appears, filtering may be appropriate. Visibility of an image feature may be desirable requiring enhancement of an image feature. Each printer makes a given spot pattern a slightly different size on output, and thus, the image will have a different darkness. Some printers will not print a single spot, or two spots lined up at one angle, but will print spots lined up at a different angle, thus changing the tone of the image. Compensation may be desirable for even day to day variations in the printing process.
It would be highly desirable to sample the output of any particular printer prior to producing a final document version, to determine the effect of any image processing technique, at that printer. In a network application, a user of an input scanner may have access to a number of printers. For example, the user may have a draft printer for composing a draft of the document, and another high speed printer for producing a large number of collated copies of the same document. Because the output quality of the draft printer and the high speed printer is not identical, it would be desirable for the user to pretest his image processing selections on the high speed printer before printing a large run of copies. Document compilation with graphics and print is not particularly fast, and repetitively producing test copies is a time consuming process.
U.S. Pat. No. 3,806,641 to Crooks demonstrates production of halftone characters from grey scale data. U.S. Pat. No. 4,149,194 to Holladay shows an arrangement which provides variable angle screens for processing the grey data. U.S. Pat. No. 4,196,454 to Warren describes a system for screening grey data and determining error between actual grey values and screened grey values to correct the output. U.S. Pat. No. 4,214,277 to Urich shows an arrangement for screening grey data. U.S. Pat. No. 4,251,837 to Janeway, III teaches an arrangement wherein an algorithm controls an image processing device to select between a grey scale screen and a black and white thresholding arrangement based on image data. U.S. Pat. No. 4,517,605 to Yokomizo shows the application of a screen to grey data wherein the screen is provided with a low spatial frequency. U.S. Pat. No. 4,517,606 to Yokomizo et al. demonstrates the use of a plurality of screens across a single image depending on the nature of the image data, showing that selected screens could be sampled across a page. U.S. Pat. No. 4,760,463 to Nonoyama et al. suggests that different scanning modes may be applied across a page at selected location. These references are incorporated herein by reference for their teachings.
The use of proofing tools is common in the photographic and printing industries, where the ability to quickly optimize the appearance of the final print is an economic necessity. These proofing devices generally include a means to isolate a portion of the image being printed, apply exposure, contrast, and/or color balance changes to the selected portion, and print it repetitively on a page. The single page is then put through the printing process and each small image evaluated for quality. A grouping of test images such as this is commonly referred to as a "ring around" in the graphic arts trade. Ring arounds display a large variety of image processing parameters in a small space and allow for side by side comparison. Devices such as step and repeat enlarging easels and variable density overlays are used in photographic darkrooms to create proof sheets.
Certain features of an image, as opposed to the entire image, are usually deemed more important or critical to image reproduction. It would therefore be desirable to sample the operation of an image processing technique with respect to a selected, critical area of an image, without repeating printing of a sheet with only a single imaging process technique applied to a single sheet.